The topology of differential amp/splitter into pentode outputs will remain the same.

Thats my whole pinto' contention, really. Or maybe it isn't...

If the current source and mirror are good as I believe they will be,
does it defeat the phase splitter? I mean, what signal does V2A get?
Both paths are held to 1/2 of a constant current. There is no UTurn
of folded current to couple differential signal between the cathodes.

Seems you are hoping for an averaged 1/(Mu-1) voltage coupling at
the cathodes to split the difference and follow at 1/2 input swing?
It just might... Not as familiar with pure voltage theory of operation.

Attached is the first schematic for another bratty push-pull amp using a pair of 6LR8 triode-pentode vertical deflection tubes per side. The 6LU8 would work just as well - same tube, different pinout. The schematic as shown has all the bell and whistles, and I'll decide how many of them I want to keep as I go on.

Options - balance control, regulated screen/not, also whether I want to use the electronic cathode bias circuit or a mess of LEDs instead (maybe Stu, just maybe). I bought the Electronic Goldmine jumbo pack of 2000 pieces of 3mm green LED (1 cent each!) so I'll be prepared if I decide to do that. I figure 5 parallel strands of 7 LEDs in series would make the nut and treat the LEDs pretty conservatively.

Since you opened this thread with the challenge for an argument...

What's the goal for this design? It will be a "one-off" with no classic sound one way or the other. With all of the SS current and voltage control you can't say it has the "tube" sound, (if there ever was really such a thing.) It's not really an experiment in using the 6LR8 in the classic tube circuit for the fun of having a "purebred" 6LR8 powered amp. All the SS stuff nullifies the attempt since the old tube amps never had all the SS regulation which would (if you believe the SS stuf has a real impact) change the distortion profile and all that goes with that ie,.feedback factor...

You should start with the pure non-SS design, measure that, then build your hybrid, measure that amp and come back with a comparison with real numbers instead of the typical nebulous HA attemps to descibe the "sound."

I can't speak for the wrench man but some of us have built enough "classic all tube" amps to last a life time. I personally have been building tube amps for about 45 years. It's time to move the tube world forward. It has been said that everything that can be done with tubes has been done. This is not so when you add some modern components. There are a few people here who can simulate and calculate "hybrid" designs and a few of us who build them. 10 years ago putting a mosfet or even a silicon rectifier in a tube amp drew a lot of flak. It it wasn't for us, we would all still be connecting a 6SN7 to a 300B to a 5U4 for ever.

The 6LU8 classic amp has been done and measured. Why do it again unless you want one.

Work on this amp is going on in the background, as it is being multiplexed with about five or six other projects. I decided as a tip o' the hat to SY to go ahead and try LED cathode bias for the output stage, and concocted some 7 deep X 5 wide arrays using green LEDs from Electronic Goldmine (2000 for $20 - 1 cent each!). Since these are standard yellow-green GaP LEDs, they'll weigh in at about 2V each, meaning that an array 7 LEDs deep will drop about 14V, which is just where I need to be to bias the pentode section of the 6LR8.

Attached is a photo of the array being hit with a current step. This was accomplised by putting the array in series with an electronic load with an internal step load feature. Quiescent current was 60mA, stepped to 150 mA. This represents a quiescent bias of 30mA/tube, then an extreme step cutting off one tube and pushing the other to 150ma. The scope picture attached shows the resulting voltage step. The overall array incremental resistance is 2.2V/90mA = 24.4 ohms. Under these conditions, each LED in the string has an incremental resistance of ~17 ohms. The LEDs are operating well within their pulsed current rating, so any decision to add more strings to the array would be based on achieving a lower incremental impedance.
The photo shows the array with a 10 ohm current sense resistor dedicated to each output tube. The array voltage shown in the scope picture was measured directly across the LEDS, not including the resistor voltage drop.

I bought a big bag full of these LED's from the Goldmine when they were on sale. I'll try to remember to bring some in here tomorrow to measure their dynamic resistance. I plan on trying a bunch of them for EL84 cathode bias. So far I have found nothing that comes close to the camera phone flash chips, but I can't get any more of them either.

LITEON LTL912SEKSA "Piranha" 3.37 Lumen Red LED

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Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......

Any of the active loads that I have ginned up so far will soundly kick the LED array's butt without breaking a sweat. However, it may be that 10-20 ohms resistance is sufficient to reap the benefits of LED cathode bias. I may just put another 10 ohm resistor in parallel with the LED array and declare victory. I was guessing that the little green LEDs were about 10 ohms apiece, but large signal excitation looks different. I may try again with a smaller current excursion to see if I get different numbers. These can't be all that different from the ones SY was using in his RLD.

I just went back to my setup with a couple of good meters and sharpened the pencil a bit. For a current excursion of 60ma to 150ma (accurately measured) I get a voltage excursion from 14.49V to 16.35V, or a delta Vee of 1.86V. This puts the resistance of the array at 20.7 ohms, and the resistance of each induividual LED at ~14.8 ohms. For both green LEDs, and red leds, your mileage may vary as to resistance. I'll have a bunch of HP/Avago LEDs coming soon. I'll try a similar measurement with them, but with a single string of 7 to keep the tedium factor down. I also have a bunch of ancient Litronix GaAsP red LEDs that might be worth a shot. Those will need more LEDs per string to reach my 14V target, though.

Then again, I alrady have a couple of active cathode bias circuits built up. When presented with a similar current transient, you can't even see any voltage excursion on the o'scope. (decisions, decisions). However, I already have an amp using similar bias networks in process. This may be an opportunity to see just how stiff you need the catode bias network in order to derive some benefit.

Last edited by wrenchone; 25th January 2011 at 12:52 AM.
Reason: New thought